1. Field of the Invention
The present invention relates to an imaging apparatus, and more specifically, to a type of imaging apparatus such as a VCR integrated with a digital video camera, in which a picture signal obtained from an imaging device is converted into a digital signal by an A/D converter so that the signal can be digitally processed and recorded on a recording medium.
2. Description of the Related Art
A known type of video camera performs clamping in such a manner that a black level of a picture signal output from an imaging device is clamped at a constant level regardless of the magnitude of the level.
Especially for a digital video camera among video cameras having the above mentioned clamping function, clamping with the following construction has been proposed: An error signal to be described below is added to an analog image signal from the imaging device to coordinate the black level of the analog signal with a target clamping level. Subsequently, the analog signal is converted into a digital signal, and a target clamping value is subtracted from the converted digital signal. Then, the resultant signal is outputted as a clamped signal. The black level during the A/D conversion process is set a little distant from a lower limit level. A signal of the black level period is extracted from the resultant digital signal. Then, the extracted signal is averaged and integrated. The integrated signal is then fed back as an error signal corresponding to the difference between the digital signal output from the A/D conversion process and the target black level. This error signal then is added to the analog signal before the A/D conversion. By the above described means, a kind of feed back clamping of the digital signal has been conducted.
In the above mentioned conventional digital video camera, when the level of the analog image signal from the imaging device is low, the analog signal is converted into a digital signal by raising the level by an amplifier or the like. However, when the level of the analog signal is raised by the amplifier, noise (random noise) carried in the black-level area in the analog signal is amplified also. When the A/D conversion of the analog signal carrying the noise is executed, the noise in the negative area which has a greater amplitude than the lower limit level of the A/D converter is totally eliminated during the A/D conversion. This type of signal in which the noise in the negative area is eliminated does not have well-balanced noise in both the positive and negative areas. As described before, the signal which is to be added to the analog signal before the A/D conversion is obtained by averaging the resultant signal and integrating the same. Therefore, when the error signal of the black-level area is extracted along with carrying the un-balanced positive-and-negative noises, and when the average value of the extracted signal is obtained subsequently, the value of the averaged signal becomes greater than the actual value because the noise component in the positive area is greater than that in the negative area. When the signal having the above mentioned incorrect value is fed back and added to the analog signal before the A/D conversion, the black level in the analog signal cannot reach the desired level, different from the set level, thus producing a problem in that a totally correct clamping cannot be performed.